Did anyone ever think how quartz watch or radio keep time so good?
It uses one special thing called crystal oscillator.
This post for Simple Crystal Tester Circuit show how to make small tester to check if crystal is working or not.
Tester are made with simple parts like transistor, resistor and capacitor.
WARNING: Making electric things can be hard.
Better to do with big person watching for safety.
What is a Crystal Tester Circuit:
Crystal Tester Circuit is easy electric circuit.
It helps to check if quartz crystal is working or not.
Quartz crystal are mostly used in many electronic things like in special oscillator.
Quartz crystal give correct and steady frequency.
Many devices use it like oscillator, clock and microcontroller.
Understanding Crystal Oscillators:
Crystal oscillator use vibrating crystal to make steady and same electric frequency.
Circuit uses quartz crystal because it is strong and can work in many ways.
Piezoelectric effect which is also call electrostriction make crystal shake when electric field touches it.
To make good crystal oscillator, crystal need correct frequency and must stay stable.
Things like crystal position, temperature and small parts on this matter.
Crystal have high Q factor so not much energy is lost in each cycle but it works like RLC circuit.
Building the Crystal Tester Circuit:
Normal meter cannot check crystal like other parts.
So this simple circuit help test crystal easily which shows if crystal is working good or broken.
Circuit Working:
Parts List:
| Component Type | Description | Quantity |
|---|---|---|
| Resistors | 100Ω, 1k, 39k (all 1/4 W CFR) | 1 each |
| Capacitors | Ceramic 0.0047µF, 1000pF, 150pF, 680pF | 1 each |
| Semiconductors | Transistors BC547 | 2 |
| Diodes OA91 | 2 | |
| LED Red 20mA 5mm | 1 | |
| Battery 12V | 1 | |
| Push Button | 1 |
Using transistor Q1 and RC parts in circuit work like Colpitts oscillator.
When we put crystal in right place Q1 starts to oscillate at crystals frequency.
This signal goes through 1000pF capacitor to two diode rectifier.
Diodes fixes the signal and send it to next transistor Q2.
When signal reaches base of Q2 it turn ON and make LED light up.
If LED glow that mean crystal is good and circuit is working fine.
But if crystal is bad Q1 does not oscillate and no signal will go to capacitor.
So Q2 stay OFF and LED also stay OFF.
This way tester show if crystal is working or not.
Formulas:
How Quartz Crystal Work with Frequency in Circuit:
Series Reactance (Xs):
Xs = R² + (Xₗₛ – X꜀ₛ)²
here:
- R is small resistance in crystal which is called ESR.
- Xₗₛ is reactance from crystals tiny coil the inductance Ls.
- X꜀ₛ is reactance from crystals tiny capacitor which is capacitance Cs.
So formula say: add square of R to square of difference between Xₗₛ and X꜀ₛ.
It does not take square root but just show how total reactance Xs change with frequency.
Capacitive Reactance (X꜀ₚ):
X꜀ₚ = -1 / (2 × π × f × Cₚ)
here:
- C or Cₛ is crystals tiny capacitance.
- f is frequency in Hertz Hz.
- π (pi) is about 3.14159.
This say: When frequency goes up capacitive reactance goes down.
Parallel Reactance (Xp):
Xp = (Xs × X꜀ₚ) / (Xs + X꜀ₚ)
This formula find parallel reactance at one frequency.
Use both Xs series and X꜀ₚ capacitive reactance.
Multiply Xs with X꜀ₚ, then divide by their sum.
Why These formulas are Important in Oscillator Circuit:
These formulas help to make good oscillator circuit with crystal.
Crystal has two special frequencies:
- fs series resonance
- fp parallel resonance
Circuit should work near these frequencies to stay stable.
How it is Build:
To build a Simple Crystal Tester Circuit follow the below mentioned connection steps:
Gather Components:
- Take all parts shown in circuit diagram.
- Check part values two times and ensure all are working good.
Prepare Transistors:
- Find base, collector, emitter of BC547 or same type transistor.
- Put Q1 and Q2 on PCB in right place.
Resistor Placement:
- Put resistors on PCB same like in diagram.
- One side of 100Ω resistor connects to Q1s collector and other side connects to positive power supply.
Diode and Capacitor Placement:
- Connect OA91 diodes to base of Q1.
- Join both diode ends and then connect that point to other transistors Q2 collector through 1000pF capacitor.
- Connect 680pF and 150pF capacitors and some in series and some in parallel with the crystal we are testing.
Connect Crystal:
- Put crystal in slot on PCB where it shown in diagram.
Connect Power Supply:
- Connect positive wire to positive power input of circuit.
- Connect negative wire to ground.
LED Connection:
- Connect LED with 100Ω resistor from Q1 collector to positive power.
Verify Connections:
- Check every wire and part again with circuit diagram.
- See that LED and capacitors are put correct way as they have polarity.
Testing:
- Turn ON power and put crystal in its place.
- If crystal is working LED will blink or turn ON.
Adjustments (if needed):
- If not working then check loose wire or wrong value parts.
- Be sure crystal and circuit both are working good.
Finalization:
- If everything is okay then we can build this on strong PCB to keep it permanent.
Conclusion:
Simple Crystal Tester Circuit is easy to make using simple parts.
It is best way to test if crystal are working or not.
By learning these basic things anyone can test crystal easy.